fix seq_length in rnn, added test for rnn

flax/nnx/nn/recurrent.py

@@ -1,3 +1,19 @@
+# Copyright 2024 The Flax Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""RNN modules for Flax."""
+
 from typing import (
     Any,
     TypeVar,
@@ -665,7 +681,7 @@ def scan_fn(carry: Carry, x: Array) -> tuple[Carry, Array]:
         scan = nnx.scan(
             scan_fn,
             in_axes=(Carry, time_axis),
-            out_axes=(Carry, (Carry, time_axis)) if slice_carry else (Carry, time_axis),
+            out_axes=(Carry, (0, time_axis)) if slice_carry else (Carry, time_axis),
             unroll=self.unroll,
         )